Various materials are manufactured in the form of a substantially continuous sheet material. For example, a web of sheet paper is manufactured by continuously depositing an aqueous suspension of fibers onto a traveling wire. Much water is drained from the wet sheet through the wire. The wet sheet is further dewatered by a press section, dried by driers and finished and smoothed by calendars. The sheet of paper produced is substantially continuous and relatively wide, generally 10 to 20 feet.
It is generally desirable to maintain certain properties of the sheet material substantially constant in the direction of the traveling sheet (machine direction or MD) and perpendicular thereto (cross machine direction or CD). However, during the manufacture of sheet material, numerous possible process and/or machine upsets may occur and it is most difficult to determine their sources. When pure machine direction variance over an entire frequency range is detected for a property of interest and decomposed into its cyclic and non-cyclic components, the sources of these components are more easily determined and corrected. However, a random variance is superposed on the pure machine direction variance, therefore the latter can not be clearly distinguished by a single detector.
Methods have been described wherein a property is obtained at a plurality of points by one or two sensors moving perpendicularly across the sheet material. Such method is taught in U.S. Pat. No.. 3,610,899 as invented by E. Dahlin and issued on Oct. 5, 1971. The data is recorded and manipulated to obtain MD and CD profiles or variances in their time domain of the property of interest and then control for example, the slice adjustment of the paper machine, accordingly. However, since the sheet material is moving in the machine direction while the sensors move across the sheet, the plurality of points are diagonally taken on the sheet. Thus, points taken on a same machine-directional line are far apart, i.e. 60 feet, and cannot detect fluctuations shorter than 120 feet (as per Nyquist Theorem). The profiles and variances are obtained after exponential weighing or filtering of the data in the time domain, which is non-indicative of the frequency composition of these variances. Also, filtering results in the loss of the contributions at higher frequencies. Furthermore, Dahlin does not distinguish between cyclic and non-cyclic MD variances.
It is an object of the present invention to provide a method and apparatus for determining on-machine, components of variances over an entire frequency range, of a property of a substantially continuous moving sheet material.
It is also an object of this invention to determine on-machine, total, machine direction, machine-directional cyclic and non-cyclic, and residual variances of a property of a moving sheet material.
It is a further object of this invention to determine residual variances at a plurality of zones across a moving sheet material.